The conventional grain trailer is equipped with a pin extending downward from the trailer underbody which is fixed to the trailer frame. This pin is received in a fifth wheel coupling on a towing, tractor truck. On usual street surfaces, the traditional tractor-trailer kingpin coupling is quite satisfactory. However, when the trailer is pulled on uneven surfaces large stresses are experienced by the kingpin and its solid mounting to the trailer underbody frame.
As fuel costs have risen, efforts to reduce operation costs for grain trucking activities have led to grain hauling trailers being designed which can be manufactured from lighter weight materials, particularly aluminum and light weight steel. When aluminum or light weight steel are used for grain trailer bodies, the mounting of the kingpin to the frame is unable to stand up to the stresses on the mounting when the trailer is pulled over uneven surfaces, particularly over grain fields, where grain trailers are often used. Twisting forces on the trailer body as it is towed over a grain field tends to heavily stress the trailer's structure, particularly when the trailer's kingpin is solidly fixed to the body. The stresses from such twisting are less well tolerated when aluminum or light weight steel is used in the stricture or when the trailer body is assembled by use of rivets instead of welding. Therefore, to afford improved fuel economy through reduction in trailer weight while maintaining the useful life of the trailer, development of structure to reduce the twisting stresses on the trailer body has been needed.